esModuleInterop should work even when compiling to esnext modules

TypeScript Version: 2.7.2
Search Terms: esModuleInterop, esnext, modules, import, export, default

Code

With this type definition:

declare function fn(): void;
declare module "external" {
  export = fn;
}

Running with:

tsc --esModuleInterop --module esnext

Produces these errors when importing:

import fn1 from 'external';       // error TS1192: Module '"external"' has no default export.
import fn2 = require('external'); // error TS1202: Import assignment cannot be used when targeting ECMAScript modules.

But, if you use commonjs modules:

tsc --esModuleInterop --module commonjs

It works as expected (because of --esModuleInterop)

import fn1 from 'external';       // works
import fn2 = require('external'); // works

Expected behavior:

It is understandable that the type checker doesn't want to pretend the import is interop'd when it's not compiling in the helpers.

But if you've specified --esModuleInterop and --module esnext the assumption from the type checker should be that an external system is applying the interop. Otherwise why would you specify --esModuleInterop?

Playground Link: https://github.com/jamiebuilds/ts-bug

Reproduce the reported imports with tsc using --esModuleInterop and --module esnext, then compare the diagnostics with the commonjs case. Trace the compiler's handling of default imports and import assignments for export = declarations; done means the esnext case follows the requested interop behavior without regressing the commonjs behavior.